In a frequency modulation/continuous wave (FM/CW) radar altimeter the noise level can vary as a function of intermediate frequency (IF). The noise level can also vary over time, temperature, and as a result of production variation. As such, the noise level is not a single number but a curve or collection of linear approximations to a curve as a function of the IF. Further, FM/CW radar altimeters continuously transmit and receive reflected transmissions. Therefore, the noise floor is typically defined from measurements containing normal ground reflection data. As such, the noise level should be determined during normal operation without disturbing the normal operation of the altimeter.